.TH std::condition_variable_any::wait_for 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::condition_variable_any::wait_for \- std::condition_variable_any::wait_for

.SH Synopsis
   template< class Lock, class Rep, class Period >

   std::cv_status wait_for( Lock& lock,                               \fB(1)\fP \fI(since C++11)\fP

                            const std::chrono::duration<Rep, Period>&
   rel_time );
   template< class Lock, class Rep, class Period, class Predicate >

   bool wait_for( Lock& lock, const std::chrono::duration<Rep,        \fB(2)\fP \fI(since C++11)\fP
   Period>& rel_time,

                  Predicate pred );
   template< class Lock, class Rep, class Period, class Predicate >

   bool wait_for( Lock& lock, std::stop_token stoken,                 \fB(3)\fP \fI(since C++20)\fP
                  const std::chrono::duration<Rep, Period>& rel_time,

                  Predicate pred );

   wait_for causes the current thread to block until the condition variable is
   notified, the given duration has been elapsed, or a spurious wakeup occurs. pred can
   be optionally provided to detect spurious wakeup.

   1) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() +
   rel_time);.
   2,3) Waiting for a specific condition to become true, can be used to ignore spurious
   awakenings.
   2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() +
   rel_time, std::move(pred));.
   3) Registers *this for the duration of this call, to be notified if a stop request
   is made on stoken's associated stop-state; it is then equivalent to return
   wait_until(lock, std::move(stoken),
                     std::chrono::steady_clock::now() + rel_time, std::move(pred));.

   Right after wait_for returns, lock is locked by the calling thread. If this
   postcondition cannot be satisfied^[1], calls std::terminate.

    1. ↑ This can happen if the re-locking of the mutex throws an exception.

.SH Parameters

   lock     - an lock which must be locked by the calling thread
   stoken   - a stop token to register interruption for
   rel_time - the maximum duration to wait
   pred     - the predicate to check whether the waiting can be completed
.SH Type requirements
   -
   Lock must meet the requirements of BasicLockable.
   -
   Predicate must meet the requirements of FunctionObject.

.SH Return value

   1) std::cv_status::timeout if rel_time has been elapsed since the beginning of this
   call, otherwise std::cv_status::no_timeout.
   2,3) The latest result of pred() before returning to the caller.

.SH Exceptions

   1) Timeout-related exceptions.
   2,3) Timeout-related exceptions, and any exception thrown by pred.

.SH Notes

   Even if notified under lock, overload \fB(1)\fP makes no guarantees about the state of the
   associated predicate when returning due to timeout.

   The effects of notify_one()/notify_all() and each of the three atomic parts of
   wait()/wait_for()/wait_until() (unlock+wait, wakeup, and lock) take place in a
   single total order that can be viewed as modification order of an atomic variable:
   the order is specific to this individual condition variable. This makes it
   impossible for notify_one() to, for example, be delayed and unblock a thread that
   started waiting just after the call to notify_one() was made.

.SH Example


// Run this code

 #include <chrono>
 #include <condition_variable>
 #include <iostream>
 #include <thread>

 std::condition_variable_any cv;
 std::mutex cv_m; // This mutex is used for three purposes:
                  // 1) to synchronize accesses to i
                  // 2) to synchronize accesses to std::cerr
                  // 3) for the condition variable cv
 int i = 0;

 void waits()
 {
     std::unique_lock<std::mutex> lk(cv_m);
     std::cerr << "Waiting... \\n";
     cv.wait(lk, []{ return i == 1; });
     std::cerr << "...finished waiting. i == 1\\n";
 }

 void signals()
 {
     std::this_thread::sleep_for(std::chrono::seconds(1));
     {
         std::lock_guard<std::mutex> lk(cv_m);
         std::cerr << "Notifying...\\n";
     }
     cv.notify_all();

     std::this_thread::sleep_for(std::chrono::seconds(1));

     {
         std::lock_guard<std::mutex> lk(cv_m);
         i = 1;
         std::cerr << "Notifying again...\\n";
     }
     cv.notify_all();
 }

 int main()
 {
     std::thread t1(waits), t2(waits), t3(waits), t4(signals);
     t1.join();
     t2.join();
     t3.join();
     t4.join();
 }

.SH Possible output:

 Waiting...
 Waiting...
 Waiting...
 Notifying...
 Notifying again...
 ...finished waiting. i == 1
 ...finished waiting. i == 1
 ...finished waiting. i == 1

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to         Behavior as published              Correct behavior
   LWG 2093 C++11      timeout-related exceptions were       mentions these exceptions
                       missing in the specification
   LWG 2135 C++11      the behavior was unclear if           calls std::terminate in
                       lock.lock() throws an exception       this case

.SH See also

   wait       blocks the current thread until the condition variable is awakened
              \fI(public member function)\fP
              blocks the current thread until the condition variable is awakened or
   wait_until until specified time point has been reached
              \fI(public member function)\fP

.SH Hidden category:
     * Pages with unreviewed LWG DR marker
